In recent years, for weight reduction in various members with the aim of improving fuel efficiency of an automobile, a reduction in thickness by achieving high strength of a steel sheet of iron alloy or the like and application of light metal such as Al alloy have been promoted. However, as compared to heavy metal such as steel, the light metal such as Al alloy has the advantage of specific strength being high, but has the disadvantage of being expensive significantly. Therefore, the application of light metal such as Al alloy has been limited to special use. Thus, in order to promote the weight reduction in various members more inexpensively and widely, the reduction in thickness by achieving high strength of a steel sheet has been needed.
The achievement of high strength of a steel sheet causes deterioration of material properties such as formability (workability) in general. Therefore, how the achievement of high strength is attained without deteriorating the material properties is important in developing a high-strength steel sheet. Particularly, a steel sheet used as an automobile member such as an inner sheet member, a structure member, or an underbody member is required to have bendability, stretch flange workability, burring workability, ductility, fatigue durability, impact resistance, corrosion resistance, and so on according to its use. It is important how these material properties and high strength property should be exhibited in a high-dimensional and well-balanced manner.
Particularly, among automobile parts, a part obtained by working a sheet material as a raw material and exhibiting a function as a rotor, such as a drum or a carrier constituting an automatic transmission, for example, is an important part serving as a mediator of transmitting engine output to an axle shaft. Such a part exhibiting a function as a rotor is required to have circularity as a shape and sheet thickness homogeneity in a circumferential direction in order to decrease friction and the like. Further, for forming such a part, forming methods such as burring, drawing, ironing, and bulging are used, and a great emphasis is placed also on ultimate ductility typified by local elongation.
Further, with regard to a steel sheet used for such a member, it is necessary to improve a property that the steel sheet is formed and then is attached to an automobile as a part and then the member is not easily broken even when being subjected to impact caused by collision or the like. Further, in order to secure the impact resistance in a cold district, it is also necessary to improve low-temperature toughness. This low-temperature toughness is defined by vTrs (a Charpy fracture appearance transition temperature), or the like. For this reason, it is also necessary to consider the impact resistance itself of the above-described steel member.
That is, a thin steel sheet for a part required to have sheet thickness uniformity such as the above-described part is required to have, in addition to excellent workability, plastic isotropy and low-temperature toughness as very important properties.
In order to achieve the high strength property and the various material properties such as formability in particular as above, in Patent Document 1, for example, there has been disclosed a manufacturing method of a steel sheet in which a steel structure is made of 90% or more of ferrite and a balance of bainite, to thereby achieve high strength, ductility, and bore expandability. However, with regard to a steel sheet manufactured by applying the technique disclosed in Patent Document 1, the plastic isotropy is not mentioned at all. On the condition that the steel sheet manufactured in Patent Document 1 is applied to a part required to have circularity and sheet thickness homogeneity in a circumferential direction, a decrease in output due to false vibration and/or friction loss caused by an eccentricity of the part is concerned.
Further, in Patent Documents 2 and 3, there has been disclosed a technique of a high-tensile hot-rolled steel sheet to which high strength and excellent stretch flange formability are provided by adding Mo and making precipitates fine. However, a steel sheet to which the techniques disclosed in Patent Documents 2 and 3 are applied is required to have 0.07% or more of Mo being an expensive alloy element added thereto, and thus has a problem that its manufacturing cost is high. Further, in the techniques disclosed in Patent Documents 2 and 3 as well, the plastic isotropy is not mentioned at all. On the condition that the techniques in Patent Documents 2 and 3 are also applied to a part required to have circularity and sheet thickness homogeneity in a circumferential direction, a decrease in output due to false vibration and/or friction loss caused by an eccentricity of the part is concerned.
On the other hand, with regard to the plastic isotropy of the steel sheet, namely a decrease in plastic anisotropy, in Patent Document 4, for example, there has been disclosed a technique in which endless rolling and lubricated rolling are combined, and thereby a texture of austenite in a shear layer of a surface layer is regulated and in-plane anisotropy of an r value (Lankford value) is decreased. However, in order to perform the lubricated rolling with a small friction coefficient over an entire length of a coil, the endless rolling is needed for preventing biting failure caused by slip between a roll bite and a rolled sheet material during rolling. However, in order to apply this technique, investment in facilities such as a rough bar joining apparatus, a high-speed crop shear, and so on is needed and thus a burden is large.
Further, in Patent Document 5, for example, there has been disclosed a technique in which Zr, Ti, and Mo are compositely added and finish rolling is finished at a high temperature of 950° C. or higher, and thereby strength of 780 MPa class or more is obtained, anisotropy of an r value is small, and stretch flange formability and deep drawability are achieved. However, 0.1% or more of Mo being an expensive alloy element is needed to be added, and thus there is a problem that its manufacturing cost is high.
Further, a study of improving the low-temperature toughness of a steel sheet has been advanced up to now, but a bainite-containing-type high-strength hot-rolled steel sheet having excellent isotropic workability that has high strength, exhibits plastic isotropy, improves hole expandability, and further achieves also low-temperature toughness has not been disclosed in Patent Documents 1 to 5.